Hereditary medullary carcinoma of the thyroid gland, a malignant neoplasm which is inherited as an autosomal dominant condition, is reliably detected by radioimmunoassay of calcitonin. The objectives of this study are to improve the methods of early detection of this neoplasm and to establish the optimum protocol for screening relatives of patients with medullary thyroid carcinoma. Measurements of calcitonin in plasma following various stimulating procedures will be performed in members of nine families already under investigation here and in members of the additional families of new index cases from this general area. Close clinical and laboratory follow-up of individuals in such families will be undertaken. Such information is important not only in providing the diagnosis of early disease, but also in establishing prognostic data on both treated and untreated individuals. Genetic linkage studies aimed at even earlier detection of this cancer will be performed on these families and other kindreds at collaborating institutions. The principles and techniques of this study will be applied to the development of early detection methods of lung cancer and other neoplasms in individuals at high risk.